PLoS Biology has an interesting paper (open access) on early metazoan evolution and the nature of the common ancestor of all metazoans.
Concatenated Analysis Sheds Light on Early Metazoan Evolution and Fuels a Modern “Urmetazoon” Hypothesis:
Following one of the basic principles in evolutionary biology that complex life forms derive from more primitive ancestors, it has long been believed that the higher animals, the Bilateria, arose from simpler (diploblastic) organisms such as the cnidarians (corals, polyps, and jellyfishes). A large number of studies, using different datasets and different methods, have tried to determine the most ancestral animal group as well as the ancestor of the higher animals. Here, we use “total evidence” analysis, which incorporates all available data (including morphology, genome, and gene expression data) and come to a surprising conclusion. The Bilateria and Cnidaria (together with the other diploblastic animals) are in fact sister groups: that is, they evolved in parallel from a very simple common ancestor. We conclude that the higher animals (Bilateria) and lower animals (diploblasts), probably separated very early, at the very beginning of metazoan animal evolution and independently evolved their complex body plans, including body axes, nervous system, sensory organs, and other characteristics. The striking similarities in several complex characters (such as the eyes) resulted from both lineages using the same basic genetic tool kit, which was already present in the common ancestor. The study identifies Placozoa as the most basal diploblast group and thus a living fossil genome that nicely demonstrates, not only that complex genetic tool kits arise before morphological complexity, but also that these kits may form similar morphological structures in parallel.
The study proposes that we used to think like this:
Now we need to think like this:
Credit: American Museum of Natural History
Triploblasts (Bilaterans) did not branch out from within an already flourishing diploblastic tree. Rather both lineages diverged from a common ancestor very early in the history of metazoan evolution and evolved similar complex characters in parallel.
I am not really qualified to comment on the research and how sound the conclusions of this study are but if this study holds up my question is what does this do to our understanding of homology? The evolution literature is full of definitions of homology. I like to think of it as the same feature modified differently in related species. So that feature was present in the most recent common ancestor of the two species. You can apply it to any feature of biological organization; bones, genes and behavior as well.
This study proposes that several similar morphological features like body axis patterning, nervous system, sensory organs were not present in the common ancestor of Bilaterans and diploblasts. So they are not homologous. They evolved independently in these two lineages. But these morphological features developed from the same ancestral genetic toolkit. Which means that these genes should be considered homologous. Off course these genetic toolkits would have evolved different expression patterns in the two lineages as well as differentiated in other ways particularly by gene duplication events, more so in the Bilaterans.
But can we consider the ancestral genetic toolkit inherited by Bilaterans and the diploblasts to be a case of molecular homology which does not translate to homology at another level (morphology)? Can we call the same gene but which is regulated differently in two species and which may control the development of similar or different features homologous or not?